Lithium secondary batteries and other nonaqueous electrolyte secondary batteries are smaller and more lightweight than conventional batteries, and they have a higher energy density and superior output density. As a result, they are now preferred for use in all kinds of portable power sources for personal computers and portable terminals and the like, and as power sources for the drives of vehicles such as hybrids.
These types of nonaqueous electrolyte secondary batteries are configured with an electrode assembly comprising a positive electrode and a negative electrode interposed by a separator, and an electrolyte that are contained in a battery case. The positive and negative electrodes each feature an electrode active material layer (specifically, a positive electrode active material layer and a negative electrode active material layer) mainly comprising an active material that can reversibly absorb and desorb a charge carrier (typically a lithium ion) at its corresponding positive or negative current collector. Furthermore, the constructed battery typically undergoes an initial charge/discharge treatment under suitable conditions to be adjusted for actual use.
However, during the initial charge/discharge treatment a solid electrolyte interface (SEI) coat is formed on the surface of the negative electrode active material by reductive decomposition of part of the nonaqueous electrolyte at the negative electrode. The presence of this SEI coat can prevent subsequent decomposition of the nonaqueous electrolyte. Therefore, the formation of a predetermined coat can prevent the deterioration of battery characteristics, protect against overcharging, and the like, and improve cycling characteristics and other battery performance. However, the charge carrier that is depleted by this reaction becomes an irreversible capacity, so the formation of the coat can cause a drop in battery capacity. Therefore, an additive that decomposes at or below the decomposition potential of the nonaqueous electrolyte and can form a coat on the surface of the negative electrode active material (hereinafter, referred to as a “coat-forming agent”) is commonly added beforehand to the nonaqueous electrolyte to form a stable coat on the surface of the negative electrode active material layer at the start of the initial charge/discharge. For example, Patent Documents 1 and 2 disclose nonaqueous electrolyte secondary batteries containing, in a nonaqueous electrolyte, an oxalato complex compound (typically lithium bis(oxalato)borate, also referred to herein as “LiBOB”) as the coat-forming agent.